package 翻硬币;

import java.util.Scanner;

public class Main {
    static int N = 110;
    static char[] str1 = new char[N];
    static char[] str2 = new char[N];

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        str1 = in.next().toCharArray();
        str2 = in.next().toCharArray();
        int res = 0;
        for (int i = 0; i < str1.length; i++) {
            if (str1[i] != str2[i]) {
                turn(i);
                turn(i + 1);
                res++;

            }
        }
        System.out.println(res);
    }

    private static void turn(int i) {
        if (str1[i] == '*') {
            str1[i] = 'o';
        } else {
            str1[i] = '*';
        }
    }
}
